TM 11-1083 Echo Box TS-280/TPS-3 1945-04-28
"After the radar set has been operating for at least 30 minutes, adjust the echo box plunger by means of the knurled handle on top of the cavity until the maximum ringing time is obtained as shown by the width of the echo box signal on the radar scopes...."
Digitized by Google Books
"April 28, 1945."
"Echo box TS-280/TPS-3 ... is a test instrument for checking over-all system performance and tuning the radio-frequency (r-f) system of radar sets operating at a frequency closed to 6000 megacycles (mc)"--Page 1